btree: simplify merge logic by using btree_last() return value

btree: simplify merge logic by using btree_last() return value

Previously btree_merge() called btree_last() only to test existence, then
performed an extra btree_lookup() to fetch the value.  This patch changes
it to directly use the value returned by btree_last(), avoiding redundant
lookups and simplifying the merge loop.

Link: https://lkml.kernel.org/r/20250826161741.686704-1-409411716@gms.tku.edu.tw
Signed-off-by: Guan-Chun Wu <409411716@gms.tku.edu.tw>
Cc: Kuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>